Organic compounds, such as benzene, toluene, methyl ethyl ketone, and ethyl acetate, are used as solvents and detergents in wide varieties of fields, such as printing, painting, coating, and electronic materials, and these compounds are partly discharged as an exhaust gas. These organic compounds may include toxic compounds, and may become the cause of a foul odor or air pollution. Thus, the exhaust gas containing such organic compounds needs to be purified. Precious metal-carried alumina catalysts, which oxidize organic compounds to remove them, have hitherto been used as exhaust gas purification catalysts.
The exhaust gas may further contain a silicon compound. For example, an organic silicone compound is excellent in heat resistance and water resistance, and is thus put to various uses. It is often coexistent in the exhaust gas. If a precious metal-carried catalyst is used for treatment of the exhaust gas containing an organic compound and a silicon compound, silicon poisons the precious metal to cause a decrease in catalytic activity (non-patent document 1). Since the silicon compounds is itself harmful, moreover, its removal is also demanded.
In order to maintain the catalytic activity even if the exhaust gas contains the silicon compound, there have been reports (patent documents 1 to 3) on methods which comprise providing an adsorbent, such as zeolite, alumina or activated carbon, containing an alkali metal or an alkaline earth metal, on a side upstream, in the direction of gas passage, of a region where the catalyst is packed (this region will be referred to hereinafter as a “succeeding stage”) (the region where the adsorbent is packed will be referred to hereinafter as a “preceding stage”) so that silicone is removed in the preceding stage before it arrives at the succeeding stage. According to these methods, however, the reactor needs to be packed with the two types of materials, thus making the structure of the reactor complicated. Furthermore, the preceding stage is poor in oxidative potential, so that high boiling substances, such as tars, are apt to accumulate there, and the accumulated substances may catch fire, causing abrupt heat generation. Thus, a single material which maintains activity for a long term is demanded.
A report has also been issued (patent document 4) on a catalyst, which has a precious metal carried on zeolite, for treatment of the exhaust gas containing a silicon compound. However, it is industrially desirable to use a carrier more inexpensive than zeolite.
As described above, there is a demand for a catalyst for treating an exhaust gas containing a silicon compound, the catalyst maintaining its activity for a long-term. Furthermore, a catalyst, which exhibits excellent performance even at a high space velocity (SV), is demanded in order to treat a large amount of an exhaust gas promptly.                Non-patent document 1: J. Catal., Vol. 86, p. 187 (1984)        Patent document 1: Japanese Unexamined Patent Publication No. 1984-147623        Patent document 2: Japanese Unexamined Patent Publication No. 1998-267249        Patent document 3: Japanese Unexamined Patent Publication No. 1997-85087        Patent document 4: Japanese Unexamined Patent Publication No. 2003-290626        